1. Field of the Invention
This invention relates generally to pseudonoise (PN) spread spectrum communication systems, and, more particularly, to apparatus and a method for generating very long composite PN codes and a method and apparatus for minimizing the time of acquisition of such very long composite codes.
2. Description of the Prior Art
The present invention may be considered to be an improvement of our U.S. Pat. No. 4,225,935. The system referenced therein combines individual PN component codes to provide a PN composite code having a code length equal to the product of the individual PN component code lengths.
The advantage in creating a composite PN code from individual component PN codes is during acquisition. The system of acquisition permits the acquisition of the first, which is usually the shortest, PN component code first. After lock-on employing code locked loops the next, which is usually the next shortest, PN component code is acquired. Thus, sequentially each of the PN component codes may be acquired until all PN component codes are acquired so as to reproduce the PN composite code.
While our previous U.S. Pat. No. 4,225,935 discloses and teaches that more than three PN component code generators may be employed in a practical PN composite code generator, theoretically the number of code generators is limited. The number of input PN code generators is limited by two factors. First, when there are three PN component code generators, the full power signal available at the receiver is not available during acquisition. It is only after acquisition and lock-on of all three component codes that the full power of the composite code is available for tracking and lock-on. For three PN component code generators, only one-half of the voltage of the receiver signal is available, thus, only one-fourth the power. This equates to a 6 dB loss and severely limits the expansion of the component PN code system beyond three. For example, it can be shown that when five component PN code generators are employed, the power is only 1/16th and the power loss attenuation has increased by a factor of 4 or is now down 12 dB.
Secondly, in our previous system it was explained that the output of the majority vote combiner (MAJ) possesses correlation component properties wherein the individual PN component codes C.sub.1, C.sub.2 and C.sub.3 each correlate with the MAJ (C.sub.1, C.sub.2, C.sub.3) composite code with a 50% correlation factor and that a composite code derived by a modulo two addition combining rule (MOD) has no correlation with the individual PN component codes.
The significance of this characteristic of MAJ and MOD composite codes is important in that while the composite code is a MAJ code it is more easily acquired by those who already know the component codes. However, it is also more easily acquired or deciphered by unfriendly forces who are reconstructing the component codes. The longer the transmitted signal is in the MAJ mode the more easily it is that it may be intercepted.
It would be extremely desirable to provide a PN composite code of any desired length without limitation which has no greater power loss during acquisition than if the code had been derived from three PN component code generators, and does not inordinately increase the acquisition time of the very long code and yet does not create a composite code more susceptible to interception.